Design an aggregation filter for stock items

- If 'aggregate=1' is sent to the stock API, aggregate the returned stock items by part and location
- Suprisingly this actually works right out of the gate

InvenTree/stock/api.py

@@ -8,6 +8,8 @@ from django_filters import NumberFilter
 from django.conf.urls import url, include
 from django.urls import reverse
 
+from django.db.models import Sum, Count
+
 from .models import StockLocation, StockItem
 from .models import StockItemTracking
 
@@ -241,6 +243,7 @@ class StockList(generics.ListCreateAPIView):
     - POST: Create a new StockItem
 
     Additional query parameters are available:
+        - aggregate: If 'true' then stock items are aggregated by Part and Location
         - location: Filter stock by location
         - category: Filter by parts belonging to a certain category
         - supplier: Filter by supplier
@@ -257,6 +260,52 @@ class StockList(generics.ListCreateAPIView):
         kwargs['context'] = self.get_serializer_context()
         return self.serializer_class(*args, **kwargs)
 
+    def list(self, request, *args, **kwargs):
+
+        queryset = self.filter_queryset(self.get_queryset())
+
+        if str2bool(self.request.GET.get('aggregate', None)):
+            # Aggregate stock by part type
+            queryset = queryset.values(
+                'part',
+                'part__name',
+                'part__image',
+                'location',
+                'location__name').annotate(
+                    stock=Sum('quantity'),
+                    items=Count('part'))
+
+            for result in queryset:
+                # If there is only 1 stock item (which will be a lot of the time),
+                # Add that data to the dict
+                if result['items'] == 1:
+                    items = StockItem.objects.filter(
+                        part=result['part'],
+                        location=result['location']
+                    )
+
+                    if items.count() == 1:
+                        result.pop('items')
+
+                        item = items[0]
+
+                        # Add in some extra information specific to this StockItem
+                        result['pk'] = item.id
+                        result['serial'] = item.serial
+                        result['batch'] = item.batch
+                        result['notes'] = item.notes
+
+            return Response(queryset)
+
+        page = self.paginate_queryset(queryset)
+        if page is not None:
+            serializer = self.get_serializer(page, many=True)
+            return self.get_paginated_response(serializer.data)
+
+        serializer = self.get_serializer(queryset, many=True)
+        return Response(serializer.data)
+                
+
     def get_queryset(self):
         """
         If the query includes a particular location,
@@ -264,7 +313,7 @@ class StockList(generics.ListCreateAPIView):
         """
 
         # Start with all objects
-        stock_list = StockItem.objects.all()
+        stock_list = StockItem.objects.filter(customer=None, belongs_to=None)
 
         # Does the client wish to filter by the Part ID?
         part_id = self.request.query_params.get('part', None)
@@ -313,6 +362,13 @@ class StockList(generics.ListCreateAPIView):
         # Pre-fetch related objects for better response time
         stock_list = self.get_serializer_class().setup_eager_loading(stock_list)
 
+        # Also ensure that we pre-fecth all the related items
+        stock_list = stock_list.prefetch_related(
+            'part',
+            'part__category',
+            'location'
+        )
+
         return stock_list
 
     serializer_class = StockItemSerializer